The present invention relates to a capacitive differential pressure detector in which fixed electrodes having the same shape and size are arranged oppositely to each other on both sides of a diaphragm displaced in accordance with a differential pressure to measure the differential pressure on the basis of the capacitance between the diaphragm and each fixed electrode, and more particularly to a capacitive differential pressure detector which can improve the static pressure span characteristic.
Now referring to FIGS. 5 and 6, an explanation will be given of an example. FIG. 5 is a sectional view of a differential pressure detector, and FIG. 6 is a sectional view of a differential pressure detecting apparatus equipped with the detector. In FIG. 5, the detector 50 has fixed electrodes 15 each having the same shape and size arranged on both left and right sides of a diaphragm 10. The fixed electrode 15 is a three-layer structure including a disk-shaped conductor plate 12 facing the diaphragm 10, a central insulating plate 13 having a square shape and a conductive plate 14 having the same square shape, these plates being stacked and bonded in order. A central hole 25 for introducing pressure is penetrated through this three-layer structure. The inner face is covered with a conductive film 27. The conductive plates 12 and 14 are communicated with each other through the conductive film 27. These two fixed electrodes 15 are bonded to the diaphragm 10 in such a form that the conductive plates are opposite to each other with respect to the diaphragm 10 through gaps 29, and also insulatively bonded to the diaphragm 10 by glass bonding portions 11 through ring-shaped supporting bodies 21 which may be either conducting or insulating material. The ring-shaped supporting bodies 21 are diametrically apart from the outer periphery of the conductive plates 12 by ring-shaped grooves 23 and are located at the peripheral portions on both sides of the diaphragm 10. In this example, the diaphragm 10, conductive plates 12, 14 and supporting body 21 are made of Si, and the insulating plate 13 is made of ceramics. Pressures P1 and P2 are introduced through the pressure-introducing center holes 25 of the left and right fixed electrodes 15, and the diaphragm 10 is displaced in accordance with the differential pressure (=P1-P2) (i.e. its central portion warps). The total capacitances C1 and C2 of the capacitors formed by the diaphragm 10 and left/right fixed electrodes 15 are taken out through lead pins C, A and lead pins C, B, respectively. Incidentally, numerals 31, 32 and 33 are vapor-deposited aluminum films (terminals for taking out capacitance).
Where the total capacitances C1 and C2 vary differentially in response to the differential pressure (=P1-P2), the output F in proportion to the differential pressure can be obtained by a known electronic circuit, where F=(C1-C2)/(C1+C2).
As seen from FIG. 6, the detector 50 is installed into a vessel mainly made up of a bottomed cylinder 51 equipped with a sealing diaphragm 59, an attaching plate 55 and a cap equipped with a sealing diaphragm 58 to constitute a differential pressure detecting apparatus. The cylinder 51 is provided at its right end face with a sealing diaphragm 59 to define a pressure chamber 62 therebetween, and the pressure chamber 62 is communicated with an internal chamber 52 inside the cylinder 51 through a center hole 60. The detector 50 is housed in the internal chamber 50. Lead pins A, B and C of the detector 50 are insulated by hermetic sealing terminals 63 and lead out to penetrate the peripheral wall of the cylinder 51. On the left side of the detector 50 (on the left side surface of the conductive plate 14, see FIG. 5), a metallic pipe 54 is provided through an insulating body (square plate member). The metallic pipe 54 is welded to an attaching plate 54 at its left end outer periphery. The attaching plate 55 is inserted into a spot facing hole on the left side of the cylinder 51 and welded thereto so that its opening is closed. The cap 56 includes a center hole 57 communicating with a pressure receiving chamber 62 which is an space inside the above sealing diaphragm 58 at the left end face. The entire internal spaces such as the pressure receiving chambers 61, 62 and internal chamber 52, which are placed between the sealing diaphragms 58 and 59, are filled with silicone oil which is non-compressive fluid. The pressure P1 acting on the sealing diaphragm is transmitted to the left side of the diaphragm of the detector 50 through the filled silicon oil whereas the pressure P2 acting on the sealing diaphragm 59 is transmitted to the diaphragm 10 (see FIG. 5).
The conventional example has a disadvantage that the static span characteristic is poor. Generally, the output characteristic of the detector when the differential pressure varies in a range of 0-100% can be substantially represented as a linear line passing an origin in case where the static pressure is used as a parameter. The output characteristic at a certain static pressure forms an slanting angle swinging or deflecting slightly upward or downward relative to the reference output characteristic established when the static pressure is zero. The degree of the swing (or deflection) is referred to as "static pressure span characteristic". The static pressure span characteristic, which can be regarded as the span error due to the static pressure, is caused by the fact that the floating capacitance formed within the detector or between the detector and a vessel incorporating the detector therein and the relative permittivity of silicon oil are varied due to static pressure. Assuming that this static pressure span characteristic is .epsilon., it can be expressed by Equation (1).
Now, in FIG. 5, areas A and B are equivalently shown, that is, Co represents the capacitance between the diaphragm 10 and the left/right conductive plate 12 when the static pressure is zero, and Cs1 represents the capacitance between the diaphragm 12 and the supporting body 21. In FIG. 6, area C is equivalently shown, that is, Cs2 represents the common capacitance between the conductive plate 14 of the right fixed electrode and the bottom of the cylinder 51 or between the conductive plate 14 of the left fixed electrode and the right side of the attaching plate. The above-noted characters are used in the Equation (1). In addition, .beta. represents variation value of the relative permittivity of the silicone oil at the static pressure P in case where the static pressure of zero is considered as a reference value. Note that the unit for the pressure P is 100 bar and the unit for the capacitance is pF.
Equation 1! EQU .epsilon.=-(Cs1+Cs.beta./Co (1+.beta.P/100)!P (1)
Now assuming that P=100 bar, .beta.=0.013, Co=50 pF, Cs1=2 pF, Cs2=1.7 pF, EQU .epsilon.=-(2+1.7)1.3.times.10.sup.-2 /50(1+1.3.multidot.1.multidot.10.sup.-4)!.multidot.1=-0.096 (%)
As understood from Equation (1), in order to improve or decrease the static pressure span characteristic .epsilon., the following measures (1) to (3) can be adopted.
(1) To decrease Cs1+Cs2 PA0 (2) To adopt a filling liquid having a smaller .beta. PA0 (3) To increase Co
However, in order to decrease Cs1 in the measure (1), the diameter of the conductive plate 12 must be reduced. This decreases Co. Hence the measure (1) contradicts the measure (3) and also is not preferable from the viewpoint of S/N ratio. The measure (2) of adopting the filling liquid other than silicone oil is not practical from the standpoint of availability or cost. The measure (3) can be achieved by increasing the area of the conductive plate 12 or decreasing the clearance or gap between the diaphragm 10 and the conductive plate 12. However, this is not actually carried out because of the dimensional restriction of the detector. The remaining measure is to decrease Cs2 in the measure (1).